Researcher Offers a Technical Perspective on Plutonium in the
Environment

Introduction

Plutonium is often inaccurately identified in the media as the "most
toxic substance known to man." Indeed, the element and its compounds are
hazardous, and measures must be employed to protect workers, the public,
and the environment. Minimizing the potential for release of plutonium is
a primary concern at TA-55. Plutonium already in the environment,
however, receives comparatively little attention.

An accurate assessment of the hazard posed by environmental plutonium is
difficult, but substantial advances in this assessment are reported.
Areas of investigation include determining the amount of environmental
plutonium, describing its distribution and migration, and evaluating its
biological and health consequences. These topics are of increasing
interest as a result of the expanding use of plutonium in mixed oxide
fuels for power generation in other countries. This article attempts to
summarize important aspects of the subject for readers of the Actinide
Research Quarterly.

Quantity, Sources, and Distribution of Environmental Plutonium

Plutonium occurs naturally as a result of neutron capture and fission of
uranium in pitch-blende ores. The resulting plutonium concentration,
about 5 x10-12 of Pu per gram of
uranium, constitutes a negligible source of environmental plutonium.

Estimates place the total amount of man-made plutonium in the environment
at 4.3 metric tons. This quantity results primarily from atmospheric
testing of nuclear weapons and to lesser extents from reprocessing of
nuclear fuel and destruction of thermoelectric generators from satellites
reentering the atmosphere.

Figure 1: Plutonium in the environment takes a number of forms and
can be distributed in a number of ways, depending on the particle size
and the dispersal mechanism.

The total alpha activity of 239Pu from aboveground nuclear
testing is estimated at 7400 tera (1012) Becquerels (TBq-see
box) and accounts for about 3.3 metric tons of plutonium. The total alpha
activity resulting from 238Pu in the environment is about 1200
TBq, or about 2 kilograms of plutonium. Approximately 92% of
environmental Pu is attributed to atmospheric testing.

A Curie (Ci) is a unit of radioactivity. One Ci corresponds to 3.7 x
1010 disintegrations per second. Because of the differences
in half-lives of various radioactive elements, a Ci represents different
amounts for different radioactive elements. For example, one Ci amounts
to radioactivity resulting from about 16.3 grams of 239Pu with
a half-life of about 24,000 years, or about 0.1 milligram of tritium,
which has a much shorter half-life. The author uses Becquerel units (Bq);
one Bq is equal to 2.7x10-11 Ci or about 4.4x10-10
grams of 239Pu (.44 billionths of a gram).

Relatively small amounts of plutonium result from accidents. About 15
kilograms
(90 TBq) of plutonium was released from the reactor at Chernobyl. The
contribution from military sources is even smaller. For example, the
aircraft accident involving nuclear weapons near Thule, Greenland, in
1968 released about 0.9 TBq of alpha activity from plutonium.

The rate of plutonium deposition in the environment has varied
substantially over the past fifty years. The largest rates were during
the period of atmospheric testing in the 1950s and early 1960s. Releases
from reprocessing facilities reached a maximum estimated rate of 70 TBq
per year during the mid 1970s, but are currently at about 0.1 TBq per
year as a result of improved facilities and procedures. A major concern
for reprocessing and storage facilities is the potential catastrophic
loss of contain-ment and a high, localized release of material.

Global distribution of plutonium is inhomogeneous. Concentrations are
high at mid-latitude zones of each hemisphere and are highest in the
northern hemisphere, where most atmospheric nuclear tests were conducted.
Maximum 239Pu activities (70 Bq-80 Bq per square meter )
appear at 35°-45°- north latitude. Activities of the isotope
are about 15 Bq per square meter at mid latitudes of the southern
hemisphere and are 1 Bq-10 Bq per square meter near the equator and
poles.

Behavior of Environmental Plutonium

Studies show that Pu exists primarily as an oxide in land deposits and in
ocean
sediments. Behavior in the environment is strongly dependent on the
physical and chemical conditions of both the material and the medium.
Important properties of the oxide are particle size and solubility.
Plutonium appears in water as ionic species of Pu(IV) and Pu(VI).

When plutonium is released into the atmosphere, its behavior depends on
the particle size and the dispersal mechanism. Stratospheric aerosols
formed by nuclear testing and satellite burnup distribute globally over a
period of years; material released by accidents typically deposit locally
within minutes or hours. Airborne redistribution of potentially
dispersible particles with geometric diameters less than 10 µm is
unlikely because such small particles readily adhere to surfaces of large
soil particles.

Processes for translocation and redistribution of environmental plutonium
are both mechanical and chemical. Vertical transport in soil is slow
compared to lateral redistribution by processes such as cultivation and
erosion by wind and water currents. Other mobilization mechanisms,
including biological transport, depend on the solubility of the plutonium
in water. In water, distribution constants, that is the fraction of the
total Pu dissolved, are in the 10-4 -10-5 range,
showing that plutonium is an insoluble solid with a solubility similar to
that of glass (SiO2).

Chemical uptake by biological systems occurs via several pathways. The
plutonium fraction transferred by root uptake of plants ranges from
10-3 to 10-5. Particle inhalation by grazing
animals is of negligible concern compared to their gastrointestinal
uptake. The fraction of ingested plutonium absorbed is approximately
10-4, and the combination of the plant uptake and animal
ingestion from plant sources shows that the fraction of deposited Pu
translocated to herbivores is 10-7 to 10-9.
Behavior in marine and fresh water systems is similar; however, Pu
concentrations in edible species (e.g., fish and crustaceans) in seawater
vary from 30 to 3000 times that of edible species in fresh water.

Human Effects of Environmental Plutonium

As with animals, incorporation of plutonium by humans also occurs
primarily
by inhalation and ingestion. Inhalation is of concern only in instances
of accidental release where local concentrations are high for a short
time. Studies of human populations give uptake fractions of
10-4 to 10-6 for ingested Pu.

The effects of plutonium on human health and longevity are the primary
concern. Biologically, Pu is classified as a radiotoxin. The risk of
cancer death is estimated to increase by 0.2% (2 in 1,000) for a person
who breathes highly contaminated air (0.1 µg of respirable oxide per
cubic meter) for one hour. Homogeneous dispersal of one kilogram of oxide
in a typical municipal water supply would result in a Pu concentration
of about 1 nanogram of Pu per liter. The increased risk of cancer death
for a person who drinks two liters of that water per day for seventy
years is estimated to be 0.01% (1 in 10,000).

Relevant Studies

Although the mission of TA-55 does not include studies on environmental
Pu, work being conducted under the Plutonium Repackaging Program is
directly related to hazard assessments for incidents involving both
facilities and weapons. Unlike Pu metal, PuO2 is a powdered
material with potential for environmental dispersal. Studies are underway
to measure the size distributions for oxides from different sources and
define the mass fractions of dispersible (< 10 µm geometric
diameter) and respirable (< 3 µm geometric diameter) particles.
Results show that the dispersible and respirable fractions of the oxide
vary by a factor of about 104 depending on the method of
preparation. A credible assessment of the dispersal hazard is possible
only if the oxide source is considered.

This article is based on "Plutonium in the Environment," LA-UR-96-1261,
by John Haschke, in the Safety Series Document, "Safe Handling and
Storage of Plutonium," Chapter 5, International Atomic Energy Agency,
April 1996.